Process of screening a shadow mask color tube

ABSTRACT

THE SHADOW MASK OF A SHADOW MASK TYPE OF COLOR PICTURE TUBE IS SHEET OF STEEL OF A PARTICULAR THICKNESS THAT IS SELECTIVELY ETCHED TO DEFINE A RECTANGULAR FIELD OF APERTURES. EACH APERTURE HAS A DIAMETER EXCEEDING THE THICKNESS OF THE BLANK AND IS SURROUNDED BY AN ELEMENTAL AREA THAT IS PARTIALLY ETCHED TO CONSTITUTE A RECESSED OR REDUCED-THICKNESS PORTION OF THE BLANK, CONCENTRIC WITH THE APERTURE. THE TUBE IS SCREENED WITH PHOSPHORS DEPOSITED BY MEANS OF A PHOTOGRAPHIC PROCESS, INVOLVING THE EXPOSURE OF A LAYER OF PHOTOSENSITIVE MATERIAL BY ACTINIC ENERGY DIRECTED TO THE SCREEN THROUGH THE FIELD OF APERTURES. THEREAFTER, THE MASK IS RE-ETCHED, REMOVING A PORTION OF THE RECESSED ELEMENTAL AREA TO ENLARGE THE MASK APERTURES TO A DESIRED SIZE RELATIVE TO THAT OF THE PHOSPHOR DEPOSITS.

s. H. KAPLAN 3,666,462

11001155 or scnsnmme A SHADOW msx coLoR TUBE May 30, 1972 Filed March 28, 1969 Inventor Sam H. Koplcm BY%WM AH ney United States Patent US. Cl. 96-361 3 Claims ABSTRACT OF THE DISCLOSURE The shadow mask of a shadow mask type of color picture tube is a sheet of steel of a particular thickness that is selectively etched to define a rectangular field of apertures. Each aperture has a diameter exceeding the thickness of the blank and is surrounded by an elemental area that is partially etched to constitute a recessed or reduced-thickness portion of the blank, concentric with the aperture. The tube is screened with phosphors deposited by means of a photographic process, involving the exposure of a layer of photosensitive material by actinic energy directed to the screen through the field of apertures. Thereafter, the mask is re-etched, removing a portion of the recessed elemental area to enlarge the mask apertures to a desired size relative to that of the phosphor deposits.

BACKGROUND OF THE INVENTION An important development in the shadow mask color television tube is the black surround screen described and claimed in US. Letters Patent No. 3,146,368, issued on Aug. 25, 1964 and assigned to the assignee of the present invention. The principle of black surround is applicable whether the phosphor deposits comprising the screen take the form of stripes, hexagons or dots. In fact, the configuration of the phosphor deposits is of no particular consequence but, for convenience, attention will be directed initially to the dot triad screen structure feature black surround. Such a screen differs from antecedent de. vices in that the phosphor dots of the triads are smaller in diameter than the apertures of the, shadow mask or color-selection electrode as .it is finally installed within the tube. This is invcontrast with the arrangement of earlier devices wherein the phosphor'tdots of the triads are slightly larger in diameter than the apertures of the shadow mask and are in essentially tangential contact with one another. By reducing the diameter of the phosphor dots, there is provided a separation or interstitial area between the dots to which may be applied a light absorbing material or pigment. Such a material is usually black and surrounds the phosphor dots from whence the name of this type of tube derives.

Such a tube presents problems in screening; in particular, it imposes the condition that the phosphor deposits be smaller in dimension than the apertures or electron transparent portions of the shadow mask or color-selection electrode.

Screening a tri-color tube is relatively easily accomplished by photographic techniques in which the image area of the tube is covered with a light sensitive material that is exposed by actinic energy directed to the screen through the apertures of the shadow mask. Such an ap proach is beneficial in that it establishes with precision the placement of the phosphor deposits on the image area of the tube in relation to the apertures of the shadow mask. Generally, the mask is in such juxtaposition to the screen that the phosphor deposits, determined through such an exposure, are the same in configuration and only slightly larger in diameter than the apertures of the ice mask. Obviously, therefore, some deviation from normal screening practices is required to satisfy the condition of the black surround tube that its phosphor deposits be smaller in size than the mask apertures.

A variety of approaches to this problem have been suggested, representative ones being the subject of US. Letters Patent. No. 3,070,441, issued on Dec. 25, 1962 and US. Letters Patent No. 3,231,380, issued on Jan. 25, 1966. These earlier efforts contemplate that the mask will be formed in conventional fashion with apertures of the size required in the final form of the mask. These apertures are then stepped down or reduced in size on a. temporary basis to have proper dimensions for use in screening. To that end, the holes are temporarily filled with a material, usually a metal that is different from the mask blank, which is removed, for example by etching, after screening has taken place. In theory, this is an acceptable solution but it raises practical problems because the filler is generally applied while the mask is still in sheet or planar form and when it is pressed into.

its required dome shape, difficulties of cracking or distortion are frequently experienced. Similar problems, giving rise to imperfections in the phosphor deposits, can be expected in most processes in which the mask is formed with large apertures to be stepped down by some temporary closure scheme. Still a further difficulty with this general approach is that the screen with temporarily filled apertures may sulfer from non-uniformities in the diameters of the mask holes.

Problems of the type described, which appear to be inherent in processes involving temporarily reducing the aperture size of the shadow mask, can be avoided by the practice of re-etching. In such a process, the mask is initially formed with apertures of the size desired for screening and after it has been used for that purpose, the mask is subjected to still another etch in order to enlarge the diameter of its holes a particular amount. A suitable re-etching process is the subject of US. Letters Patent No. 2,961,313, issued Nov. 22, 1960 and assigned to the assignee of the present invention. It contemplates that the mask blank have a coating of resist on each of its faces with aperture patterns in each coating to 'give access to selectively exposed portions of the mask blank in order that those portions may be removed by an etchant. The holes of the coatings correspond with the initial size desired for the apertures in the mask. After the mask has been used for screening and while it still retains the resist coatings, it is immersed a second time in an etch bath so that portions of the mask immediately surrounding the initially formed holes, and lying beneath the resist coating, may be attacked by the etchant and removed to enlarge the apertures to a desired final size.

The present invention, while generally similar to the re-etch process discussed in the last paragraph, is an improvement in that it avoids the inconvenienceof re-etching with the resist coatings in situ and further in teaching a practical process by which aperture sizes that are substantially larger than the thickness of the blank may be formed by re-etching without seriously reducing the strength of the mask structure.

Accordingly, it is an object of the invention to provide a novel shadow mask for a color picture tube and a unique process for screening such a tube.

It is another object of the invention to provide an improved process for screening a shadow mask type of color picture tube featuring black surround.

It is a more specific object of the invention to provide an improved process for screening a shadow mask type of color picture tube to attain phosphor deposits that are smaller in size than the mask apertures, as required of tubes featuring black surround or post-deflection acceleration.

SUMMARY OF THE INVENTION The process of preparing the screen section of a shadow mask type of color picture tube, in accordance with the invention, comprises selectively etching a metal blank of a given thickness to define a field of apertures individually having dimension in the plane of the blank at least equal to the blank thickness and individually circumscribed by an elemental area of the blank similar in configuration to the aperture and only partially etched to constitute a recessed portion that is concentric with but has dimensions large compared to those of its associated aperture. A plurality of deposits of difierent phosphor materials are applied to the screen of the tube by steps which, for each of the materials, comprise covering the screen area with a layer of photosensitive material and then exposing that layer through the field of apertures of the mask to actinic energy to determine the location on the screen of deposits of the particular material being applied. After screening, the mask is re-etched to remove a portion to the recessed elemental areas contiguous to the mask apertures in order to enlarge the apertures a predetermined amount relative to the size of the phosphor deposits. Finally, the re-etched mask is secured and spacedin essentially parallel relation to the screen of the tube.

For the conventional form of color tube in' which the phosphor deposits define a multiplicity of dot triads, the mask apertures are round and have an initial diameter exceeding the blank thickness but of proper size for screening. The process under consideration permits the screening of the tube while, at the same time, affording to the re-etched mask necessary mechanical strength even though the diameter of the re-etched or final apertures of the mask is substantially greater, two or three times as great as the mask thickness.

BRIEF DESCRIPTION OF THE DRAWINGS The features of the present invention which are believed to be novel are set forth with particularity in the appended claims. The invention, together with further objects and advantages thereof, may best be undestood by reference to the following description taken in connection with the accompanying drawing, in the several figures of which like reference numerals identify like elements, and in which:

FIG. 1 is a fragmentary view of the screen section of a shadow mask type of color tube and also a shadow mask in screening relation to the image area of the tube;

FIGS. 2, 3 and 4 are fragmentary views of the aperture mask as it appears in different stages of the screening process; and

FIG. 5 represents another form of mask that may be used in practicing the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Color tubes of the type under consideration may have circular or rectangular envelope configurations at the faceplate or screen section which includes the image area. do this area, there is a repeating sequence of phosphor deposits representing the three primary colors because commercial television, as presently practiced, is an additive type of system. As stated above, the phosphor deposits take any of a variety of forms such as stripes, hexagon, circular dots or the like. The specific configuration of the envelope or of the elemental phosphor deposits is of.

no particular moment but, for convenience, it will be assumed that the tube under consideration is a rectangular one having a mosaic screen defined by a multiplicity of dot triads each of which constitutes a dot of green phosphor, a dot of blue phosphor and a dot of red phosphor.

in FIG. 1, the screen 9 has phosphor deposits represented by circles and the legends G, B and R symbolically 1 l designate those of the deposits which are formed of green, blue and red phosphors, respectively. It will be observed that the phosphor dots are not in tangential contact. This is because they are smaller than usual and, therefore, are separated from one another providing an area of the screen around each such dot which may accommodate a light-absorbing material or pigment in accordance with the teachings of US Letters Pat. No. 3,146,368, although for convenience of illustration, that pigment has not been represented in the drawing. Moreover, a shadow mask tube featuring post-defiection-focus employs the same screen arrangement whether or not it also has such a pigment surrounding the phosphor dots.

superposed over and in close spaced relation to the screen or image area of the tube is a shadow mask 10, only a portion of which is shown in FIG. I. It is a relatively thin metallic sheet or blank of a given thickness, usually formed from cold rolled steel having a thickness of approximately 6 or 7 mils. This sheet has a multiplicity of apertures 11 arranged in a field with dimensions and configurations corresponding to the dimensions and configurations of the image area. As shown in the enlarged view of FIG. 3, each individual aperture of the mask has dimensions in the plane of the blank at least equal tov the thickness of the blank and each aperture is circumscribed by an elemental area of the blank having a configuration similar to that of the aperture but only partially etched to constitute a recessed or reduced-thickness portion of the mask blank that is concentric with, but has dimensions large compared to those of, its associated aperture. Where the phosphors are deposited in circular dots, as has been assumed for the screen under consideration, each aperture 11 is circular and its initial diameter D is at least equal to, but usually slightly larger than, the thickness t of the mask blank. The concentric recess 11a has a diameter D which, in a practical embodiment of the invention, is approximately two and one-half times the dimensions D The recessed portion of the mask blank immediately contiguous to aperture 11 resembles a generally flat dish having a sharply rising edge flange. It has a thickness t; of less than one-half the thickness of the blank, preferably being about one-third to one-quarter of the blank thickness. Obviously, the effective hole size for the mask in the condition of FIG. 3 is represented by the dimension D which is chosen in relation to the size of the phosphor deposits desired for the screen of the tube. More particularly, for the case under consideration in which the screen has a multiplictiy of dot triads, aperture 11 is dimensioned initially to achieve the desired diameter of the phosphor dots which is attained photographically by light focused upon the screen through the apertures of the mask.

Before considering the screening process as such, it is appropriate to describe a method of forming a shadow mask with an arrangement of apertures as shown in FIG. 3. It is most convenient to use metal stock, such as cold rolled steel, into which apertures may readily be formed by etching with precision, not only as to their location within a desired field or pattern, but also as to dimension. Procedures for accurately and controllably etching apertures in such a sheet are well within the skill of the art and are described, for example, in the following [1.8. Letters Patent: 2,750,524, issued on June 12, 6; 2,762,149, issued on Sept. 11, 1956; and 2,961,313, issued on Nov. 22, 1960.

The process involves well known techniques of photoprinting by which a pattern is established on a sheet followed by chemical milling or etching through which that pattern is developed. More particularly, the mask blank is first coated on both sides or surfaces with a photosensitive material 12 having such properties that its solubility in a given solvent is influenced by exposure to actinic en-. ergy. Preferably, an organic photosensitive material is employed of such nature that those portions thereof that are exposed to ultraviolet light are rendered insoluble.

Conveniently, polyvinyl alcohol sensitized with ammonium dichromate may be used as the photosensitive material because it is normally soluble in water but becomes insoluble upon exposure to ultraviolet light. Alternatively, other resist materials such as dichromated fish glue may be employed. Having applied coatings 12 to both surfaces of blank 10, a design is projected on each such surface by ultraviolet light to transfer that design by the exposure. Specifically, the pattern of the design for the mask under consideration shields each aperture portion 11a on one surface of the mask blank from exposure and similarly shields each portion 11b on the obverse side of the blank. The designs are projected from accurately determined positions to the end that the shielded portions of the photosensitive coating overlaying areas 111: and 11b are in coaxial alignment throughout the entire aperture field. Aside from these shielded portions associated with the intended position of each aperture that is to be formed in the mask blank, the entirety of the blank is exposed on both surfaces and the image resulting from such exposure is developed by spraying, washing or otherwise treating the surfaces of the blank with the solvent for the photosensitive coating. In this treatment, the exposed portions of the photosensitive material remain in situ while theunexposed portion-or remainder of the coatings is washed off. Accordingly, the surface areas of the blank at the intended location of each aperture portion is stripped of its coating and is thus exposed as illustrated in FIG. 2. These exposed elemental areas of the mask blank are then subjected to an etch bath of ferric chloride of suitable concentration. While the exposed elemental areas of each side of the mask blank may be etched individually, it is preferred to etch both sides of the mask blank simultaneously.-

By controlling the parameters of the etching process, including in particular the concentration of the etchant and the porcessing time, the desired aperture 11 and its circumscribing recessed or reduced-thickness portion 11a are formed with precisely determined dimensions. None of the mask blank is removed in the portions that are interpersed with apertures 11 and recesses 11a on both faces of the blank due to the protectiveelfect of the photoresist and yet the thickness 1 of the recessed elemental areas may be readily dimensioned to the end that the mask exhibits adequate mechanical strength for preserving and protecting the desired apertures 11 while, atthe same time, preparing the mask for a convenient and quick reetch process, to be discussed hereafter, in orderto enlarge the diameter of apertures llafter the screening shall have been completed.

When the field of apertures, individually constructed as indicated in FIG. 3, has been. formed, the mask is appropriate for use in screening the picture tube. In the first screening step, if the tube is of the dark surround variety. the image area of the tube is covered with a photosensitive resist composition in which light-absorbing material, such as manganese carbonate, is carried in suspension as described in US. Letters Patent No. 3,365,- 292. This resist is of the type that it is rendered soluble upon exposure to actinic energy and the coated screen is exposed three times with each exposure being made through shadow mask 10. In this process step, actinic energy, such as ultraviolet light, is able to reach the screen only through apertures 11 so that each exposed area of the sensitized coating is a circle of essentially the same diameter D of its associated mask aperture. In each of the three exposure steps, the light source is positioned to simulate one of the three electron guns of the tube and in this fashion latent images are established of the elemental areas of the screen that are intended to receive phosphor deposits. Washing the exposed screen with a solvent for that photosensitive resist develops these images by removing all exposed portions of the coating. What remains on the screen is a black surround material as a suspension in an organic material that will be removed in bakeout.

The screen is now in condition to receive the phosphor 6 deposits and each material is applied by means of a similar photographic technique in which the image area is first covered with a layer of photosensitive resist in which one. of the three phosphor materials is held in suspension. In this instance, however, an opposite type of resist is employed, that is to say, a resist of the kind that is rendered insoluble by exposure to ultraviolet light such as polyvinyl alcohol sensitized with ammonium dichromate. The screen is next selectively exposed through the apertures of the shadow mask from a light source positioned to simulate the electron gun of the color tube assigned to excite the particular phosphor in process. After the exposure, latent images of the desired deposits of the particular phosphor will have been made and they are developed by washing the screen with a solvent for the resist, such as water, leaving deposits of the phosphor properly located on the screen area relative to the black surround material. Such deposits may or may not overlap the black area. The remaining two phosphors are applied in like manner and in each instance the location of the exposing light source is adjusted to simulate the appropriate electron beam of the tube in process. At this juncture, the screen will have been completed so far as the deposits of phosphor and the black surround material are coiicerned. It may now be lacquered and aluminized in the usual way so that the phosphor dots have a backing layer of an electron permeable, conductive and light reflecting metal, usually aluminum. After aluminizing, the screen is ready for bakeout although this step is usually taken with the shadow mask in position within the faceplate section of the tube.

When the screening has been accomplished, the aperture mask is removed from the faceplate section of the tube and is re-etched. The re-etching takes place preferabliy without any protective resist coating on the surfaces of the mask blank and causes the diameter of apertures 11 to be enlarged to the dimension D shown in FIG. 4. At the same time, the thickness of the mask blank is reduced to the dimension 1, and the thickness of the recessed elemental areas is likewise reduced to the value I C This results from the fact that the etchant attacks all surfaces of the blank since none havev been protected against the etchant. Of course, the parameters ofv the re- 1 u etching step are determined so that the final dimensionD of aperture 11 is properly related to the diameters of the phosphor dots that have previously been established on the screen of the tube. More specifically,.and as required for either a black surround or post-deflection tube, the

- final aperture diameter D; is large compared with the Phosphor dots which have a dimension somewhat larger than D The mask is now essentiallyconventional, having apertures of normal size, and is suitable for final installation into the faceplate section of thetube in parallel relation to the screen area, preparatory to bakeout.

Representative dimensions for a shadow mask of the type considered in connection with FIGS. 2-4, inclusive, are as follows:

Mils Inlt al blank thickness t 7 Initial aperture diameter D 9 Recessed portion diameter D 22 Initial thickness 1; of recessed portion 2 /2 Final aperture diameter D, 16 Final mask thickness r 5 Final thickness t of recessed portion 1 /2 The described structure is attractive in that the apertures of the mask may be formed with an initial diameter of a desired carefully controlled size. They may be uniform in size or, if desired, may be weighted in accordance with proposals that the aperture diameter decrease with radial distance from the center of the mask. Additionally, the re-etching to enlarge the mask apertures to a desired final size may be accomplished in controlled fashion and quickly so that the mask retains adequate mechanical strength. A re-etch time of 1 /2 minutes has been used successively. Since the recessed portion 111: surrounding the apertures initially formed in the mask blank has a reduced thickness, less than half the original blank thick ness, the aperture enlargement resulting from the re-etch causes a minimal reduction in the thickness of the mask blank intervening the apertures of the mask. The mask as finally completed, with apertures of the diameter D is sufliciently strong for use in the commercial production of color tubes. Of course, added strength maybe real ized by increasing the weight of the starting mask material but the described process makes it possible to minis mize the need to use heavier stock.

It is not necessary that the apertures of the 'mask'be circular; in the arrangement of FIG. 5, mask 10 has a series of slots 20 which are required for atube' having phosphor deposits in the form of strips. The slots have an initial width t and an enlarged width dimension 'attained after re-etching. Again, the enlargement is achieved quickly in the re-etching step by removing portions of the recessed or reduced thickness elemental areas adjacent slots 20. 7

While particular embodiments of the invention have been shown and described, it will be obvious to those skilled in the art that changes and modifications may be made without departing from the invention in its broader aspects, and, therefore, the aim in the appended claims is to cover all such changes and modifications as fall within the true spirit and scope of the invention. I claim: 1. The process of preparing the shadow mask and the screen section of a shadow mask type of color picture tube which comprises: 7 l

forming photoinsolubilized etch-resistant coatings on both sides of a mask blank having a certain thickness wherein said coatings have corresponding apertures where the said blank is to be etched to form the shadow mask holes, the diameter or size of said apertures on one side of said blank being thicker than said blank thickness but less than one-half the diameter or size of the associated aperture on the opposite side of said blank to which it is to be etched to form'a single hole in said blank;

etching both sides of said blank through said apertures:

' largest diameter or size at least twice as large as the largest diameter or size of the small opening, the etched openings meeting at a; point approximately one- -third of the distance from the outside surface of the blank side having the small openings as compared to the total'blank thickness to form a surface in saidhole having a generally flat dish with a sharp rising edge flange circumscribing said small p g .i' removing said etch-resistant coatings from both sides of said rhask'blank after the etching process step but before the re-etching process step; applying to said screen area of said tube a plurality of a deposits of different phosphor materials applied, for each of said materials, by means of a photographic technique in which said ssreen area is first covered by a'laycr of photosensitive material and is then exposed through said apertures of said blank with actinic energy to determine the location on said screen area of deposits of the phosphor material being applied; thereafter re-etching said mask blank stripped of said etch-resistant coatings to remove a portion, but less than all, of said shoulder to enlarge said apertures of said blank a predetermined amount r the size of said phosphor deposits;

and'finally securing said mask in spaced essentially parallel relation to said screen area of said tube.

relative to 2. The process in accordance with claim 1 inwhich' References Cited 7 UNITED STATES PATENTS 3,519,869 7/1970 "Kuniyoshi 313-92 B X 3,574,013 4/19 7l Erantzen 3l392 B X CHARLES L. 'BOWERS, 1a., Primary Examiner Us. (:1. X.VR. 9 -53 313 ss s, 92 B, 92 PD 7 

